Intel’s 22-nm Trigate Transistors Exposed

Last week Intel had their Q1 conference call for financial analysts, and revealed that the 22-nm Ivy Bridge parts would make up 25% of their shipment volume in the second quarter of this year. That means that a good quantity will already will have shipped, and we managed to track some down in Hong Kong a few weeks ago. Of course we got in touch ASAP and the parts duly arrived, and they were the real thing.

Fig. 1 Intel Xeon E3-1230V2 Server CPU

We obtained samples of Xeon E3-1230 v2 CPUs, which are four-core, 3.3 GHz, 64-bit parts intended for the server market. Here is a die photo of the transistor level, with annotations from Intel’s Ivy Bridge launch yesterday:

Fig.2 Intel Xeon E3-1230V2 Die

A quick cross-section reveals that Intel have stayed with the nine metal layers used in the last two generations:

Fig. 3 Intel Xeon E3-130V2 General Structure

A closer TEM image (Fig. 4) shows the lower metal stack and a pair of multi-fin NMOS and PMOS transistors. This section is parallel to the gate, across the fins, and we can see the contact trenches and metal levels M1 up to M5.

We have to digress here a little to explain what we’re looking at. A typical TEM sample is 80 – 100 nm thick, to be thin enough to be transparent to the electron beam and at the same time have enough physical rigidity so that it does not bend or fall apart.

Here we are trying to image structures in a die with a gate length of less than 30 nm; so if we make a sample parallel to the gate, and if the sample is aligned perfectly along the centre of the gate, then it will contain the gate plus at least part of the source/drain (S/D) silicon and contacts on either side.

That is what we see above – I have labeled the gate and contact stripes, and we have PMOS on the right and NMOS on the left. The tungsten-filled contacts obscure parts of the gate, but we can clearly see that the PMOS S/D fins have epitaxial growth on them, and the fins have an unexpected slope – a little different from Intel’s tri-gate schematic shown last year –see Fig.5.

Fig. 5 Intel Schematic of Tri-Gate Transistor

If we zoom in a bit further into the PMOS gate (Fig. 6), we can see how the gate wraps over the fin, and the rounded top of the fin. The thin dark line adjacent to the fin is the high-k layer and just above that is a mottled TiN layer that is likely the PMOS work-function material, as in the 32-nm and 45-nm parts.

Fig. 6 TEM Image of PMOS Gate and Fin Structure

Fig. 7 shows a section of an NMOS transistor. There is a ‘ghost’ of the contact behind the gate, but the gate structure itself looks similar to the PMOS, with the exception of the work-function material just above the high-k layer (as expected).

Fig. 7 TEM Image of NMOS Gate and Fin Structure

Fig. 8 gives me an opportunity to show off our new TEM – we have recently purchased an FEI Osiris machine, which upgrades our capability considerably. Here we have a lattice image of a fin in an NMOS transistor; the diamond-like layout of the pattern of dots is actually created by the columns of atoms in the silicon crystal lattice. This tells us that the sample is oriented in the <110> direction, which given that silicon has a face-centred cubic structure in which equivalent planes are at right angles, means that the channel direction is also <110>.

Fig. 8 TEM Lattice Image of NMOS Fin Structure

To fully understand what we’re looking at, of course, we need to see what’s happening in the orthogonal direction, along the fin and cross-sectioning the gate – as in Fig. 9. This shows an array of PMOS transistors over a single fin, four functional gates and two dummy gates at the ends of the fin. Again the TEM sample is thick compared with the feature size, so we are seeing the gate on the side(s) of the fin, not just the top. The fin ends have the same taper as in Figs 6 and 7.

Fig. 9 TEM Image of PMOS Transistors

As announced by Intel, there is embedded SiGe in the source/drains, although not etched to the <111> planes as in the 32- and 45-nm product. It also looks as though the tops of the gates have been etched back and back-filled with dielectric, and the contacts are self-aligned as in memory chips.

Zooming in on the PMOS transistor in Fig.10, the image is a bit fuzzy, but the SiGe is clearly in a rounded cavity with no facets on the top, though there are facets on the sides of the fin (see fig. 4).

Fig. 10 TEM Image of PMOS Transistor

Looking at the NMOS equivalent (Figs. 11 and 12), we see a similar structure – there seems to be an epitaxial interface, and the silicide(?) seems to protrude slightly above the fin.

Fig. 11 TEM Image of NMOS Transistors

Fig. 12 TEM Image of NMOS Transistors

It is hard to say much about the gates here, either NMOS or PMOS, because of the sample thickness problem; we are viewing a slice that includes the gate on both sides of the fin and the fin itself. Fortunately we have images of gate metal over STI and they are less confusing.

Figure 13 is a composite image of NMOS and PMOS gates so that the differences are highlighted. The dark line surrounding the gate structures is the Hf-based high-k, and within that are the two work-function materials, likely TiN for PMOS and TiAlN for NMOS. (The columnar structure of the PMOS TiN is visible in the right half of the image.)

Fig. 13 Composite TEM Image of NMOS/PMOS Gates

The fill has been changed from TiAl in the earlier parts to tungsten. It is more prominent in the NMOS gates than the PMOS, because the PMOS structure includes both work-function metals, whereas the TiN has been etched out of the NMOS gates. At the 45-nm node Intel used tensile tungsten in the contacts to apply channel stress – have they transposed this to the gates in the 22-nm process?

Just to finish up, so that this is still a blog, not a paper (I don’t want to go on too long) – fig. 14 shows a sample delayered to expose the transistors, and imaged on a tilt angle. Both the gates and the fins show up nicely, and we can actually see tiny spikes of SiGe in the PMOS source/drains. The small pillars in between the fins in the NMOS areas are residual bits of contact metal. I think it’s a cool image!

Fig. 14 Tilt SEM Image of NMOS/PMOS Transistors

We are just getting into the full scope of the analysis, so likely more to come in the next few weeks!I’m still tweeting as @ChipworksDick, for those that way inclined..